This is a known problem in Xara X1. While this problem was greatly reduced in Xtreme, it is still occasionally reported.

The CPU usage should stop if the font gallery is closed and re-opened. It starts using 100% CPU if you scroll the gallery fast enough to scroll past font previews that haven't yet been drawn (so the thumbnail is just a grey rectangle). So you can stop it using 100% CPU by slowly (e.g. one "page" at a time) scrolling through the whole list, giving it a chance to redraw the images.